1. Field of the Invention
This invention relates to non-interlace television receivers, and more particularly, to a double-scanning non-interlace television receiver with a vertical aperture correction circuit.
2. Description of the Prior Art
Present television systems, such as the NTSC system, employ an interlace scanning line method in which one picture or frame is comprised of two interlaced fields of scanned lines. The interlace system was developed to increase the number of scanning lines as much as possible in order to reduce undesirable flicker in the picture and yet remain within the limited frequency band available for transmission.
One suggested apparatus for improving the sharpness of a television picture uses a delay circuit for one horizontal period of a scanning line to emphasize, in a vertical direction, the high frequency components of the video signal to be displayed.
When a video signal which is displayed as a vertical line having a significant change between two adjacent points in the level or brightness thereof is emphasized by such a high frequency emphasizing circuit, the video signal so emphasized has a preshoot at the point before the change in level, where the level is under-emphasized, and an overshoot at the point after the change in level, where the level is over-emphasized. It is to be appreciated that the two points between which the change in level or brightness occurs are positioned on successive horizontal scanning lines. In an interlace scanning system, both video signals for the two fields of scanning lines are emphasized, so that each level corresponding to the preshoot and overshoot is displayed on two lines. It is preferable for maximum picture sharpness that each level corresponding to the preshoot and overshoot be displayed on one line, however.